1. Field of the Invention
The present invention elates to a method for producing dies for extruding ceramic honeycomb structural bodies which are used for heat exchangers, filters and catalyst carriers for purifying exhaust gases from internal combustion engines, more particularly such a type of die having an abrasive-resistant material coated on the surface thereof by chemical vapor deposition (CVD).
2. Related Art Statement
Heretofore, ceramic honeycomb structural bodies have been used for carriers of catalysts for purifying exhaust gases from internal combustion engines, filters for removing soot, rotary heat exchangers for recovering heat, or the like. Methods for producing a die used for producing such a type of honeycomb structural bodies by extrusion are described, for example, in JP-A-60-145,804 and JP-A-61-69,968, wherein techniques of subjecting the extrusion dies to chemical vapor deposition treatments are described. However, a chemical vapor deposition treatment is expensive in cost and uneconomical for treating a single die, and thus it is desired to treat as many extrusion dies as possible simultaneously in one operation.
Therefore, applicants' assignee previously filed a Japanese Patent Application No. 2-81,315 wherein a technique of simultaneously treating two dies is proposed. Namely, the chemical vapor deposition treatment is carried out into effect by providing two sheets of die members 4,4 in a space 16 for the CVD defined by a circumferential side wall 18 and a bottom plate (perforated plate) 17 of a device for vapor deposition, providing a rotatable tube 6 for supplying a raw material gas at the central axis of the device, and passing a raw material gas for CVD from holes 7 of the rotatable tube 6 in the direction of the arrow B towards extrusion grooves of the die members 4, 4, as shown in FIGS. 3 and 4. The whole structure of the vapor deposition device used is shown in FIG. 5 wherein the portion encircled by a dotted line (die member-accommodating portion) corresponds to the device of FIG. 3, and a portion other than the die member-accommodating portion forms also a space for CVD (CVD space) defined by the perforated plate 17 and the circumferential side wall 18 of the device similarly as in the die member accommodating portion for allowing the raw material gas to flow therethrough to perform another CVD treatment, for example, of other parts.
In the above described technique of simultaneously treating two sheets of die member, the raw material gas is discharged from the discharge holes 7 of the rotatable pipe 6 in the space 16 and flowed out from the CVD space 16 in a gas outlet space 9 via six holes 12 of eight holes 12 formed on the circumferential side wall 18 except those 2 holes formed behind the die members 4,4, as shown in FIG. 4. Therefore, a large amount of the raw material gas is exhausted from the gas outlet space 9 without participating in the chemical vapor deposition which is performed in the CVD space 16 to decrease an efficiency of utilizing the raw material gas. The tendency of decreasing the gas utilization efficiency is more remarkable, if the die member-non-accommodating portion of the CVD device shown in FIG. 5 is not used for another CVD treatment. Moreover, because the bottom plates 17 have many perforation holes 13, the raw material gas in the CVD space 16 is likely to flow to the vertical direction along a course of the arrows B and E shown in FIG. 3 and flow out therefrom to the exterior. Accompanying these phenomena, the flow of the raw material gas in the CVD space 16 is disturbed to occasionally form an uneven vapor deposited film of an abrasion resistant material of non-homogeneous thickness and/or quality on the die members 4,4. The non-homogeneity of the film likely occurs particularly at the rear side of the extrusion grooves of the die members. If the die having such uneven vapor deposited film is used to actually produce honeycomb structural bodies by extrusion operation, there often arise disadvantages in that the extruded bodies are bent or have undulations on the surface. Moreover, in the case when another die members 4,4 are arranged in other CVD space of the CVD device shown in FIG. 5 to form a multi stage type CVD device in order to increase the number of the die members that can be treated or vapor deposited simultaneously in one treatment, the thicknesses of the vapor deposited films become uneven depending on the position of the stages due to the aforementioned reasons.